Case Class Instantiation From Typesafe Config - scala

Question

Suppose I have a scala case class with the ability to serialize into json (using json4s or some other library):
case class Weather(zip : String, temp : Double, isRaining : Boolean)
If I'm using a HOCON config file:
allWeather {
BeverlyHills {
zip : 90210
temp : 75.0
isRaining : false
}
Cambridge {
zip : 10013
temp : 32.0
isRainging : true
}
}
Is there any way to use typesafe config to automatically instantiate a Weather object?
I'm looking for something of the form
val config : Config = ConfigFactory.parseFile(new java.io.File("weather.conf"))
val bevHills : Weather = config.getObject("allWeather.BeverlyHills").as[Weather]
The solution could leverage the fact that the value referenced by "allWeather.BeverlyHills" is a json "blob".
I could obviously write my own parser:
def configToWeather(config : Config) =
Weather(config.getString("zip"),
config.getDouble("temp"),
config.getBoolean("isRaining"))
val bevHills = configToWeather(config.getConfig("allWeather.BeverlyHills"))
But that seems inelegant since any change to the Weather definition would also require a change to configToWeather.
Thank you in advance for your review and response.

Answer 1

typesafe config library has API to instantiate object from config that uses java bean convention. But as I understand case class does not follow those rules.
There are several scala libraries that wrap typesafe config and provide scala specific functionality that you are looking for.
For example using pureconfig reading config could look like
val weather:Try[Weather] = loadConfig[Weather]
where Weather is a case class for values in config

Answer 2

Expanding on Nazarii's answer, the following worked for me:
import scala.beans.BeanProperty
//The #BeanProperty and var are both necessary
case class Weather(#BeanProperty var zip : String,
#BeanProperty var temp : Double,
#BeanProperty var isRaining : Boolean) {
//needed by configfactory to conform to java bean standard
def this() = this("", 0.0, false)
}
import com.typesafe.config.ConfigFactory
val config = ConfigFactory.parseFile(new java.io.File("allWeather.conf"))
import com.typesafe.config.ConfigBeanFactory
val bevHills =
ConfigBeanFactory.create(config.getConfig("allWeather.BeverlyHills"), classOf[Weather])
Follow up: based on the comments below it may be the case that only Java Collections, and not Scala Collections, are viable options for the parameters of the case class (e.g. Seq[T] will not work).

Answer 3

A simple solution without external libraries, inspired from playframework Configuration.scala
trait ConfigLoader[A] { self =>
def load(config: Config, path: String = ""): A
def map[B](f: A => B): ConfigLoader[B] = (config, path) => f(self.load(config, path))
}
object ConfigLoader {
def apply[A](f: Config => String => A): ConfigLoader[A] = f(_)(_)
implicit val stringLoader: ConfigLoader[String] = ConfigLoader(_.getString)
implicit val booleanLoader: ConfigLoader[Boolean] = ConfigLoader(_.getBoolean)
implicit val doubleLoader: ConfigLoader[Double] = ConfigLoader(_.getDouble)
}
object Implicits {
implicit class ConfigOps(private val config: Config) extends AnyVal {
def apply[A](path: String)(implicit loader: ConfigLoader[A]): A = loader.load(config, path)
}
implicit def configLoader[A](f: Config => A): ConfigLoader[A] = ConfigLoader(_.getConfig).map(f)
}
Usage:
import Implicits._
case class Weather(zip: String, temp: Double, isRaining: Boolean)
object Weather {
implicit val loader: ConfigLoader[Weather] = (c: Config) => Weather(
c("zip"), c("temp"), c("isRaining")
)
}
val config: Config = ???
val bevHills: Weather = config("allWeather.BeverlyHills")
Run the code in Scastie

Answer 4

Another option is to use circe.config with the code below. See https://github.com/circe/circe-config
import io.circe.generic.auto._
import io.circe.config.syntax._
def configToWeather(conf: Config): Weather = {
conf.as[Weather]("allWeather.BeverlyHills") match {
case Right(c) => c
case _ => throw new Exception("invalid configuration")
}
}

Answer 5

Another tried-and-tested solution is to use com.fasterxml.jackson.databind.ObjectMapper. You don't need to tag #BeanProperty to any of your case class parameters but you will have to define a no-arg constructor.
case class Weather(zip : String, temp : Double, isRaining : Boolean) {
def this() = this(null, 0, false)
}
val mapper = new ObjectMapper().registerModule(DefaultScalaModule)
val bevHills = mapper.convertValue(config.getObject("allWeather.BeverlyHills").unwrapped, classOf[Weather])

Answer 6

Using config loader
implicit val configLoader: ConfigLoader[Weather] = (rootConfig: Config, path: String) => {
val config = rootConfig.getConfig(path)
Weather(
config.getString("zip"),
config.getDouble("temp"),
config.getBoolean("isRaining")
)
}